Multicomponent optical body

ABSTRACT

Multilayer polymeric films and other optical bodies are provided. The films, which have at least three layers of different composition in the optical repeating unit, reflect light in a first portion of the spectrum while transmitting light in a second portion of the spectrum, exhibit improved reflectivities at oblique angles, and can be designed to suppress one or more higher order harmonics of the main reflection band.

FIELD OF THE INVENTION

[0001] The present invention relates generally to a multilayer polymericfilms that reflect light in a first portion of the spectrum whiletransmitting light in a second portion of the spectrum, and inparticular to a reflective polymeric film having at least three layersof different composition in the optical repeating unit.

BACKGROUND OF THE INVENTION

[0002] The use of multilayer films comprising multiple alternatinglayers of two or more polymers to reflect light is known and isdescribed, for example, in U.S. Pat. No. 3,711,176 (Alfrey, Jr. et al.),U.S. Pat. No. 5,103,337 (Schrenk et al.), WO 96/19347, and WO 95/17303.The reflection and transmission spectra of a particular multilayer filmdepends primarily on the optical thickness of the individual layers.Optical thickness is defined as the product of the actual thickness of alayer and its refractive index. Accordingly, films can be designed toreflect infrared, visible or ultraviolet wavelengths λ_(M) of light byappropriate choice of optical thickness of the layers in accordance withthe following formula:

λ_(M)=(2/M)*D_(r)  (Formula I)

[0003] wherein M is an integer representing the order of the reflectedlight, and D_(r) is the optical thickness of an optical repeating unit(also called multilayer stack) comprising two or more polymeric layers.Accordingly, D_(r) is the sum of the optical thicknesses of theindividual polymer layers that make up the optical repeating unit. Byvarying the optical thickness of an optical repeating unit along thethickness of the multilayer film, a multilayer film can be designed thatreflects light over a wide range of wavelengths.

[0004] From Formula I, it can also be seen that a multilayer film oroptical body which is designed to reflect light in a first region of thespectrum may have higher order reflections in a second region of thespectrum. For example, a multilayer film designed to reflect infraredlight will also have higher order reflections in the visible region ofthe spectrum. Specifically, a multilayer film designed to have a firstorder reflection (M=1) at 1500 nm may have higher order reflections atabout 750 nm (M=2), 500 nm (M=3), 375 nm (M=4), etc. A film designed toreflect infrared light of even longer wavelengths may have even morehigher order reflections in the visible region. Thus, for example, amultilayer film having a first order reflection at 2000 nm will havehigher order reflections at 1000 nm, 666 nm, 500 nm, 400 nm, etc. Thesehigher order reflections are undesirable in many applications (e.g.,window films) because they impart an iridescent appearance to the filmwhere a transparent, colorless appearance is preferred. Therefore, inorder to design a multilayer film that reflects light over a firstregion of the spectrum (e.g., the infrared region) but does not reflectlight over a shorter wavelength region (e.g., the visible region), atleast two, and preferably at least three higher order reflections needto be suppressed.

[0005] U.S. Pat. No. 5,103,337 (Schrenk et al.) teaches that an infraredreflecting multilayer film having an optical repeating unit withpolymeric layers A, B and C arranged in an order ABC, is capable ofsuppressing at least two successive higher order reflections when theindex of refraction of polymeric layer B is chosen to be intermediate tothat of polymeric layers A and C. In a particular embodiment of the filmdescribed therein, the optical repeating unit is formed by arranginglayers A, B and C in an ABCB pattern. By selecting polymeric materialsA, B and C such that the refractive index of material B equals thesquare root of the product of the refractive index of materials A and C,and by setting the optical thickness ratio for material A and C to ⅓ andthat of material B to ⅙, at least three higher order reflections can besuppressed. Similar teachings are found in Thelen, A., J.Opt.Soc. Am.53, 1266 (1963). However, one disadvantage of this design is that theamount of reflection of incident light with the first order harmonicdecreases with increasing angle of incidence. A further disadvantage ofthis design is that the suppression of the three higher orderreflections also decreases with increasing angle of incidence. Thislater result is particularly undesirable in applications such as windowfilms where the infrared reflective film is used to shield a room frominfrared sunlight, since the sunlight will frequently be incident atangles substantially away from the normal (particularly in the springand summer when the sun is high in the sky).

[0006] U.S. Pat. No. 5,540,978 (Schrenk) teaches a multilayer polymericfilm that reflects ultraviolet light. In one embodiment, the filmincludes first, second, and third diverse polymeric materials arrangedin a repeating unit ABCB. In another embodiment, the layers are arrangedin the repeating unit ABC.

[0007] WO 96/19346 teaches reflective films that are made out of anoptical repeating unit of alternating layers A and B, where A is abirefringent polymeric layer and B can be either isotropic orbirefringent. The reference notes that, by matching the index ofrefraction between both layers along an axis that is perpendicular tothe surface of the film, the dependency of reflection on angle ofincidence can be greatly reduced. However, the reference does not teachhow these results can be extended to multilayer optical systems havingthree or more layer types in the repeating unit (e.g., films with ABC orABCB repeating units). Such a system would be highly desirably, bothbecause of the improvement in reflectivity at oblique angles it wouldafford, and because the additional layer or layers in the repeating unitcould be used to impart better mechanical properties to the system. Forexample, one of the additional layers could be an optical adhesive thatwould reduce the tendency of the other layers to delaminate.Furthermore, while WO 96/19346 mentions infrared reflective films, itdoes not describe how an IR film can be made that will not suffer fromhigher order reflections in the visible region of the spectrum (e.g., ifthe first order reflection is at 1200 nm or more).

[0008] There is thus a need in the art for a multilayer film or otheroptical body that exhibits a first order reflection band for at leastone polarization of electromagnetic radiation in a first region of thespectrum (e.g., in the infrared, visible or ultraviolet regions of thespectrum) but can be designed to suppress at least the second, andpreferably also at least the third, higher order harmonics of the firstreflection band. In particular, there is a need in the art for amultilayer film or optical body that has a first reflection band in theinfrared region of the spectrum but that exhibits essentially no higherorder reflection peaks in the visible region of the spectrum.

[0009] There is also a need in the art for a film or other optical bodyhaving three or more layer types in its optical repeating unit, and forwhich the reflectivity of the film (e.g., toward infrared radiation)remains essentially constant, or increases, at non-normal angles ofincidence.

[0010] These and other needs are met by the films and optical bodies ofthe present invention, as hereinafter described.

SUMMARY OF THE INVENTION

[0011] In one aspect, the present invention provides films and otheroptical bodies which exhibit a first order reflection band for at leastone polarization of electromagnetic radiation in a first region of thespectrum, while suppressing at least the second, and preferably also atleast the third, higher order harmonics of the first reflection band.

[0012] In another aspect, the present invention provides a multilayeroptical film having at least three different layer types in its optical;repeating unit, and for which the % reflection of the first orderharmonic remains essentially constant, or increases, as a function ofangle of incidence. This may be accomplished, for example, by forming atleast a portion of the optical body out of polymeric materials A, B, andC which are arranged in a repeating sequence ABC, wherein A hasrefractive indices n_(x) ^(A), n_(y) ^(A), and n_(z) ^(A) along mutuallyorthogonal axes x, y, and z, respectively, B has refractive indicesn_(x) ^(B), n_(y) ^(B), and n_(z) ^(B) along axes x, y and z,respectively, and C has refractive indices n_(x) ^(C), n_(y) ^(C) andn_(z) ^(C) along axes x, y, and z, respectively, where axis z isorthogonal to the plane of the film or optical body, wherein n_(x)^(A)>n_(x) ^(B)>n_(x) ^(C) or n_(y) ^(A)>n_(y) ^(B)>n_(y) ^(C) andwherein n_(z) ^(C)≧n_(z) ^(B) and/or n_(z) ^(B)≧n_(z) ^(A). Preferably,at least one of the normalized differences 2(n_(z) ^(A)−n_(z)^(B))/(n_(z) ^(A)+n_(z) ^(B)) and 2(n_(z) ^(B)−n_(z) ^(C))/(n_(z)^(B)+n_(z) ^(C)) is less than about −0.03.

[0013] Surprisingly, it has been found that, by designing the film oroptical body within these constraints, at least some combination ofsecond, third and fourth higher-order reflections can be suppressedwithout a substantial decrease of the first harmonic reflection withangle of incidence, particularly when the first reflection band is inthe infrared region of the spectrum. Films and optical bodies made inaccordance with the present invention are therefore particularly usefulas IR mirrors, and may be used advantageously as window films and insimilar applications where IR protection is desired but goodtransparency and low color are important.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present invention is described in detail by way of referenceto the following drawings, without, however, the intention to limit theinvention thereto:

[0015]FIG. 1 is a schematic illustration of an optical repeating unit inaccordance with the present invention;

[0016]FIG. 2 is a graph of reflectance as a function of angle ofincidence for the repeat unit of FIG. 1 when the indices of refractionarc related as specified in Case 1 of Table I;

[0017]FIG. 3 is a graph of reflectance as a function of angle ofincidence for the repeat unit of FIG. 1 when the indices of refractionare related as specified in Case 2 of Table I;

[0018]FIG. 4 is a graph of reflectance as a function of angle ofincidence for the repeat unit of FIG. 1 when the indices of refractionare related as specified in Case 3 of Table I;

[0019]FIG. 5 is a graph of reflectance as a function of angle ofincidence for the repeat unit of FIG. 1 when the indices of refractionare related as specified in Case 4 of Table I;

[0020]FIG. 6 is a graph of reflectance as a function of angle ofincidence for the repeat unit of FIG. 1 when the indices of refractionare related as specified in Case 5 of Table I;

[0021]FIG. 7 is a graph of reflectance as a function of angle ofincidence for the repeat unit of FIG. 1 when the indices of refractionare related as specified in Case 6 of Table I;

[0022]FIG. 8 is a graph of reflectance as a function of angle ofincidence for the repeat unit of FIG. 1 when the indices of refractionare isotropic; and

[0023]FIG. 9 is a graph of measured transmittance as a function ofwavelength for a sample in which the repeat unit has the index ofrefraction relationship specified in Case 1 of Table I.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The following definitions and conventions are used throughout thedisclosure:

[0025] Multilayer film: a film comprising an optical repeating unitdesigned to reflect light over a particular range of wavelengths. Themultilayer film may contain additional layers between the opticalrepeating units which may or may not be repeated throughout themultilayer film.

[0026] Optical repeating unit: a stack of layers arranged in aparticular arrangement which is repeated across the thickness of amultilayer film.

[0027] In-plane axes: two mutually perpendicular axes disposed in theplane of the film. In the present application, these axes will typicallybe designated as the x-axis and the y-axis.

[0028] Transverse axis: an axis that is perpendicular to the plane ofthe film. In the present application, this axis will typically bedesignated as the z-axis.

[0029] The index of refraction for light polarized along a particularaxis will typically be denoted as n_(i), wherein i indicates theparticular axis (i.e., n_(x) denotes the index of refraction for lightpolarized along the x axis). The normalized index difference is thedifference between refractive indices divided by the average of thoseindices. This accounts for dispersion (i.e., changes in refractive indexwith wavelength).

[0030] Negative birefringence: the situation in which the index ofrefraction along the transverse axis is less than the index ofrefraction along one or both in-plane axes (n_(z)<n_(x) and/or n_(y))

[0031] Positive birefringence: the situation in which the index ofrefraction along the transverse axis is greater than the index ofrefraction along one or both in-plane axes (n_(z)>n_(x) and/or n_(y))

[0032] Isotropic: the situation in which the indices of refraction alongthe x, y and z-axes are the same (i.e., n_(x)=n_(y)=n_(z))

[0033] Infrared region: 700 nm to 2500 nm

[0034] Visible region: 400 nm to 700 nm

[0035] Optical thickness: the ratio defined as:$f^{k} = \frac{n^{k}*d^{k}}{\sum\limits_{M = 1}^{1}{n^{M}d^{M}}}$

[0036] wherein f^(k) is the optical thickness of polymeric layer k, 1 isthe number of layers in the optical repeating unit, n^(k) is therefractive index of polymeric layer k, and d^(k) is the thickness ofpolymeric layer k. N^(M) is the refractive index of the M^(th) polymericlayer, and d^(M) is the thickness of the M^(th) polymeric layer. Theoptical thickness ratio of polymeric layer k along an optical axis j isdenoted as f_(j) ^(k) and is defined as above but with replacement ofn^(k) with the refractive index of polymeric material k along axis j(n_(j) ^(k)).

[0037] Skin layer: a layer that is provided as an outermost layer.Typically, skin layers in the films and optical bodies of the presentinvention will have a thickness between 10% and 20% of the sum of thephysical thicknesses of all optical repeating units.

[0038] Monotonically varying thickness of an optical repeating unitalong a multilayer film: the situation in which the thickness of theoptical repeating unit either consistently decreases or consistentlyincreases across the thickness of the film (e.g., the thickness of theoptical repeating unit does not show an increasing trend along part ofthe thickness of the film and a decreasing trend along another part ofthe thickness of the film).

[0039] In accordance with one embodiment of this invention, a film orother optical body is provided which has an optical repeating unitcomprising a multilayer stack containing m layers, where m is an integerof 4 or more. Such an optical repeating unit includes polymeric layersA, B and C, which are preferably arranged in an optical repeating unithaving the layer sequence ABCB. The optical thickness ratio for each ofthe polymeric layers preferably have the values f_(x) ^(a)=⅓, f_(x)^(b)=⅙ and f_(x) ^(c)=⅓ and/or f_(y) ^(a)=⅓, f_(y) ^(b)=⅙ and f_(y)^(c)=⅓, wherein the second, third, and forth reflection harmonics aresuppressed.

[0040] A schematic drawing of such a repeating unit is shown in FIG. 1.However, the values for the optical thickness ratios are not a necessarycondition of this embodiment, as other optical thickness ratios can alsoresult in suppression of some combination of second, third andforth-order reflection harmonics.

[0041] In the films and other optical bodies made in accordance with thepresent invention, it is preferred that at least one of the differencein indices of refraction between layers A and B along the z-axis (n_(z)^(a)−n_(z) ^(b)) and the difference in indices of refraction betweenlayers B and C along the z-axis (n_(z) ^(b)−n_(z) ^(c)) is negative.More preferably, at least one of these normalized differences is lessthan or equal to −0.03, and most preferably, at least one of thesenormalized differences is less than or equal to −0.06. In a particularlypreferred embodiment of the present invention, the optical repeatingunit is designed such that at least one of these normalized differencesis less than 0 and preferably is less than or equal to −0.03, and suchthat the other difference is less than or equal to 0. Most preferably,both differences are less than 0. It has been found that such designsyield the most robust performance and exhibit an increase in thereflection of p-polarized light (and therefore, in total reflection)with increasing angle of incidence.

[0042] It is also possible to design a film or other optical body inaccordance with the present invention which has an optical repeatingunit in which both differences are substantially 0, i.e., wherein theabsolute value of the normalized differences is preferably less thanabout 0.03. When both differences are substantially 0, there will belittle or no decrease of the infrared reflection of p-polarized lightwith the angle of incidence.

[0043] According to a still further embodiment of the present invention,one of the differences in refractive index between layers A and B acrossthe z-axis is of opposite sign to the difference in refractive indexbetween layers B and C across the z-axis. In this embodiment, it ispreferred that either the difference that is less than 0 has the largestabsolute value or that the absolute value of both differences issubstantially equal. Films and other optical bodies in accordance withthis embodiment will have a substantially constant or increasingreflectance for p-polarized light with increasing angle of incidence,yielding a substantially increasing reflectance for unpolarized incidentlight with increasing incidence angle.

[0044] While the above described embodiments all yield optical repeatingunits which substantially suppress some combination of the second, thirdand fourth higher-order reflection harmonics, and for which there iseither an increase of the p-polarized infrared reflection with anincrease in angle of incidence of the light or a substantially constantp-polarized infrared reflection when the angle of incidence increases,it has been found that, when both differences are substantially largerthan 0 or when one of them is substantially larger than 0 and the otheris essentially 0, a substantial and unacceptable decrease of infraredreflection of p-polarized light is noticed as angle of incidenceincreases, leading to a decrease in reflectance for randomly polarizedincident light. An example of this is illustrated in FIG. 8.

[0045] The behavior of the infrared reflection with angle of incidenceis depicted for each of the above described embodiments for opticalthickness ratios f_(x) ^(a)=⅓, f_(x) ^(b)=⅙ and f_(x) ^(c)=⅓ and/orf_(y) ^(a)=⅓, f_(y) ^(b)=⅙ and f_(y) ^(c)=⅓, in FIGS. 2-7. FIG. 2 showsthe infrared reflection for p-polarized and s-polarized light with angleof incidence for an optical repeating unit wherein the difference n_(z)^(a)−n_(z) ^(b) is −0.14 and the difference n_(z) ^(b)−n_(z) ^(c) is+0.13 and in which the optical repeating unit is otherwise designed suchthat the in-plane index relationships are in accordance with the presentinvention. In FIG. 3, the difference n_(z) ^(a)−n_(z) ^(b) is +0.15, andthe difference n_(z) ^(b)−n_(z) ^(c) is −0.17. The behavior of both thep-polarized light and s-polarized light are shown since both componentsof light determine the total amount of light reflected for the typicalinstance of unpolarized incident light. However, the present inventionallows for control (increasing reflectance with increasing angle ofincidence) of the p-polarized component of the reflected light in anovel and unexpected way.

[0046] The present invention is further illustrated by reference to thecomparative example shown in FIG. 8 with the examples of FIGS. 2 or 3.FIG. 8 illustrates how the reflectance changes with angle of incidencefor repeat units in which the transverse index relationships amongcomponents A, B and C are not controlled in accordance with the presentinvention. FIG. 8 illustrates the typical variation of the amount ofreflection of p-polarized and s-polarized light from the repeat stack,with the angle of incidence of light, where the difference n_(z)^(a)−n_(z) ^(b) is −0.15 and the difference n_(z) ^(b)−n_(z) ^(c) is−0.13 . The materials A, B and C are each isotropic; their refractiveindices are the same along all three axes. The overall reflectance forunpolarized incident light (the average of P and S-polarizedreflectance) substantially decreases with increasing incidence angle. Asseen from FIG. 8, there is a substantial decrease of the reflection ofp-polarized light when the angle of incidence increases. In the exampleshown in FIG. 8, the components of the repeating unit A, B and C, areisotropic, meaning that the in-plane and transverse indices are equal.By contrast, a further embodiment in accordance with the presentinvention for which both differences are very small compared to the inplane index differences, exhibits substantially constant reflection ofinfrared p-polarized light as the angle of incidence is varied (FIG. 4).FIG. 4 shows a typical variation of the amount of reflection ofp-polarized and s-polarized light from the repeat stack, where thedifference n_(z) ^(a)−n_(z) ^(b) is 0 and the difference n_(z)^(b)−n_(z) ^(c) is 0 (Table 1 Case 3).

[0047]FIG. 5 shows an embodiment wherein n_(z) ^(a)−n_(z) ^(b) is −0.13and wherein the difference n_(z) ^(b)−n_(z) ^(c) is −0.15 . (Table 1Case 4). As can be seen from this figure, the infrared reflection forp-polarized light in this embodiment strongly increases with anincreasing angle of incidence. Similarly, FIG. 6 shows the reflectancebehavior for embodiments wherein n_(z) ^(a)−n_(z) ^(b) is −0.13 and thedifference n_(z) ^(b)−n_(z) ^(c) is 0 (Table 1 Case 5), and FIG. 7 showsthe reflectance behavior for embodiments wherein n_(z) ^(a)−n_(z) ^(b)is 0 and the difference n_(z) ^(b)−n_(z) ^(c) is −0.13. As can be seenfrom these figures, the infrared reflection of p-polarized light in thesystems of FIGS. 6 and 7, as with the system of FIG. 5, increases withangle of incidence.

[0048]FIG. 9 illustrates the measured spectra of a film made inaccordance with the present invention. The film stack uses a 4 layerrepeat unit of the type ABCB wherein A is PEN, B is coPEN and C is PMMA.The stack is composed of a total of 15 repeat units. The overallreflectance of the average of S-polarized and P-polarized light,increases with incidence angle. The refractive indices for polymers A,Band C used in this example are substantially identified by those in Case1 Table 1 shown below. In this example polymeric layers A, B and C haverefractive index values such that n_(x) ^(b)=(n_(x) ^(a)n_(x) ^(c))^(½)and/or n_(y) ^(b)=(n_(y) ^(a)n_(y) ^(c))^(½) (square-root condition)while keeping the following in-plane optical thickness ratios: f_(x)^(a)=⅓, f_(x) ^(b)=⅙ and f_(x) ^(c)=⅓ and/or f_(y) ^(a)=⅓ f_(y) ^(b)=⅙and f_(y) ^(c)=⅓. The P-polarized reflectance at 60 degrees issubstantially the same (with a change in wavelength location) as it isfor normally incident light. As a result, the overall reflectance forunpolarized incident light (the average of P and S-polarizedreflectance) substantially increases with increasing incidence angle.

[0049] In a further aspect of the present invention, the polymericlayers A, B and C have, along at least one in-plane axis, refractiveindices which differ from each other. In particular, the refractiveindices are such that the refractive index of polymeric layer B isintermediate to that of layers A and C along at least one in-plane axis.Furthermore, since polymeric layer A has the highest refractive indexalong at least one in-plane axis, the indices of refraction are inaccordance with at least one of the relations specified in Formulas IIand III:

n_(x) ^(a)>n_(x) ^(b)>n_(x) ^(c)  (Formula II)

n_(y) ^(a)>n_(y) ^(b)>n_(y) ^(c)  (Formula III)

[0050] In the case where only one of Formulas I and II are fulfilled(e.g., where n^(a)>n^(b)>n^(c) along only one in-plane axis), therelationship along the other in-plane axis may be of any kind;preferably, however, the indices of refraction are substantially equalalong this axis. Films and other optical bodies made in accordance withthis embodiment will substantially reflect light polarized along thefirst in-plane axis and will substantially transmit light polarizedalong the other in-plane axis, leading to a reflective polarizer in thewavelength range encompassed by the first harmonic reflection.

[0051] In an especially preferred embodiment of the present invention,the optical repeat unit is designed such that the refractive indexrelationship in accordance with this invention is fulfilled along bothin-plane axes, thereby yielding an optical repeating unit capable ofreflecting light independent of its plane of polarization. IR mirrorsmade in accordance with this embodiment reflect infrared radiation butare substantially transparent to visible radiation, i.e., higher orderreflections in the visible region of the spectrum are suppressed.

[0052] By adjusting the optical thickness ratios along the particularin-plane axis that has the index of refraction for polymeric layer Bintermediate that of polymeric layer A and polymeric layer C, at leasttwo higher order reflections for infrared light having its plane ofpolarization parallel to that particular in-plane axis can besuppressed. It is, however, preferred that the index of refraction forpolymeric layer B be intermediate that of polymeric layers A and C alongboth in-plane axes and, by adjusting the optical thickness ratios alongboth in-plane axes, an infrared reflective mirror can be obtained forwhich at least two successive higher order reflections are suppressed.Such an infrared reflective mirror will be substantially transparent inthe visible region and will be free of color (e.g., iridescence).

[0053] In another especially preferred embodiment of the presentinvention, the refractive indices for polymeric layers A, B and C aresuch that n_(x) ^(b)=(n_(x) ^(a)n_(x) ^(c))^(½) and/or n_(y) ^(b)=(n_(y)^(a)n_(y) ^(c)) ^(½) (square-root condition), and layers A, B, and Chave the following in-plane optical thickness ratios: f_(x) ^(a)=⅓,f_(x) ^(b)=⅙ and f_(y) ^(a)= ⅓, f_(y) ^(b)=⅙ and f_(y) ^(c)=⅓. Such aconfiguration will consist of a repeat unit consisting of material Awith optical thickness f_(x) ^(a)=⅓, followed by material B with f_(x)^(b)=⅙, followed by material C with f_(x) ^(c)=1/3, finally followed bymaterial B again with f_(x) ^(b)= ⅙. Such a repeat cell is symbolicallyrepresented as ABCB. Embodiments of the present invention which havethis repeat cell are capable of suppressing second, third and fourthorder reflections for normally incident light. Accordingly, a reflectivefilm designed according to this embodiment can be used to reflectinfrared light up to about 2000 nm without introducing reflections inthe visible region of the spectrum.

[0054] The particular refractive index relationships in accordance withthe present invention can be obtained by appropriate selection of thepolymeric materials used for each of the individual layers. The presentinvention typically requires that at least one of the polymeric layersA, B and C is a birefringent polymer. One or more of the other layersmay be birefringent as well, or the other layers may be isotropic.Depending upon the particular polymer or polymer blend selected for apolymeric layer and on the processing conditions used to produce theoptical repeating unit, a polymeric layer can be negativelybirefringent, positively birefringent, or isotropic. The following table(Table I) shows embodiments that can yield optical repeating units inaccordance with the present invention (in particular, the especiallypreferred embodiment described above, or other embodiments wherein f_(x)^(a)=⅓, f_(x) ^(b)=⅙ and f_(x) ^(c)=⅓ and/or f_(y) ^(b)=⅓, f_(y) ^(b)=⅙and f_(y) ^(c)=⅓, but for which the square-root condition is notsatisfied) leading to increasing levels of reflectance with increasingangles of incidence.

[0055] The relationships among the refractive indices along the z-axisdescribe the invention for the more general case in which the in-planeindex of refraction of polymer B is intermediate to that of polymers Aand C, and the in-plane index of refraction of polymer A is greater thanthat of polymer C, with at least two successive higher-order harmonicsbeing suppressed. It is to be understood that Table I shows generalrelationships among refractive indices for a given set of in-planerefractive indices, and that the amount of the difference in the indexof refraction along the z-axis among the polymeric layers A, B and C inaccordance with this invention will depend on the amount ofbirefringence of the birefringent layer(s). TABLE I Example Cases (ABCBConfig.) in which Reflectivity Increases with Incidence Angle Ny and/orSign (N_(y) ^(a) − Sign(N_(z) ^(a) − Sign(N_(z) ^(b) − Case Nx Nz TypeN_(y) ^(b)) Sign (N_(y) ^(b) − N_(y) ^(c)) N_(z) ^(b)) N_(z) ^(b)) 1|N_(z) ^(a) − N_(z) ^(b)| ≧ |N_(z) ^(b) − N_(z) ^(c)| A 1.78 1.49 B(+)(+) (−) B 1.63 1.63 I (+) (+) C 1.50 1.50 I 2 |N_(z) ^(a) − N_(z) ^(b)|≦ |N_(z) ^(b) − N_(z) ^(c)| A 1.78 1.78 I (+) (+) B 1.63 1.63 I (+) (−)C 1.50 1.80 B(−) 3 A 1.78 1.50 B(+) (+) 0 B 1.63 1.50 B(+) (+) 0 C 1.501.50 I 4 A 1.78 1.50 B(+) (+) (−) B 1.63 1.63 I (+) (−) C 1.50 1.78 B(−)5 A 1.78 1.50 B(+) (+) (−) B 1.63 1.63 I (+) 0 C 1.50 1.63 B(−) 6 A 1.781.63 B(+) (+) 0 B 1.63 1.63 I (+) (−) C 1.50 1.76 B(−)

[0056] The physical thickness of the individual polymeric layers A, Band C is generally selected so as to obtain a desired optical thicknessratio as explained above. Accordingly, the particular physical thicknessof a layer is not a primary concern (of course, the physical thicknesspartly defines the optical thickness and the optical thickness of theoptical repeat unit determines the wavelengths of the reflected light).However, the physical thickness of polymeric layers A, B and C istypically less than about 0.5 micrometers.

[0057] It is further preferred in the films and optical devices of thepresent invention that the normalized refractive indices between thepolymers A, B and C are at least about 0.03 along an in-plane axis forwhich the refractive index relationship is in accordance with Formula IIor Formula III. Thus, it is preferred that the normalized differencesbetween n_(x) ^(a), n_(x) ^(b) and n_(x) ^(c) are at least about 0.03and/or that the normalized differences between n_(y) ^(a), n_(y) ^(b)and n_(y) ^(c) differ from each other by at least about 0.03.

[0058] The especially preferred embodiment described above in whichpolymeric layers A, B and C have refractive indices such that n_(x)^(b)=(n_(x) ^(a)n_(x) ^(c))^(½) and/or n_(y) ^(b)=(n_(y) ^(a)n_(y)^(c))^(½), and having the in-plane optical thickness ratios f_(x)^(a)=⅓, f_(x) ^(b)=⅙ and f_(x) ^(c)32 ⅓ and/or f_(y) ^(a)=⅓, f_(y)^(b)=⅙ and f_(y) ^(c)=⅓, will suppress the second, third and forthreflection harmonics for normally incident light. When the incidentlight is non-normally incident, these higher-order reflection harmonicsmay become unsuppressed to a degree, depending on the polarization ofthe incident light, and the refractive index relationships among thein-plane and the z-axis of each polymeric material. Indeed, the degreeto which higher-order harmonics, which are suppressed at normalincidence, provide reflectance at higher incidence angles can besubstantial, leading to films which become colored or which arereflective to one polarization state at high incidence angles. Suchoptical properties can be controlled by specifying the indexrelationships described in Table I above. Each case will have increasingreflection of the first-order harmonic with increasing incidence angle,but will have differing amounts of increase in reflectance for thehigher-order reflection harmonics (from zero at normal angle) withincreasing angle of incidence. For example, Case 3 of Table I willresult in negligible increase in reflectance with increasing angle(reflectance remains nearly zero), while Case 4 of Table I will exhibita significant increase in reflectance for the higher-order harmonicswith increasing angle of incidence.

[0059] Similarly, refractive index dispersion (changes in the in-planeaxis and transverse axis refractive indices, with wavelength) can resultin a degree of non-suppression of higher-order harmonics in certainwavelength region, even though the conditions for complete higher-orderharmonic suppression are met in other wavelength regions. The degree towhich refractive index dispersion can change the degree of higher-orderharmonic suppression depends on the dispersion characteristic of theparticular polymeric material comprising the repeat cell; certainpolymers have greater dispersion than others. Such effects can beameliorated by choices of polymers A, B and C, and by techniques such asdesigning the f-ratios to “best match” the required values forhigher-order harmonic suppression across the wavelength range ofinterest. Indeed, both the effects of index dispersion and angle ofincidence (described above) on causing a degree of non-suppression ofhigher-order harmonics can be minimized through polymer choice and“best-match” f-ratio design. The best match f-ratio design may include avariation in f-ratio distribution as a function of total repeat unitthickness. The distribution of repeat unit thickness may likewise beoptimized.

[0060] Embodiments of the present invention for which the polymericmaterials A, B and C have in-plane optical thickness ratios f_(x)^(a)=⅓, f_(x) ^(b)=⅙ and f_(x) ^(c)=⅓ and/or f_(y) ^(a)=⅓, f_(y) ^(b)=⅙and f_(x) ^(c)=⅓, but which do not satisfy the condition n_(x)^(b)=(n_(x) ^(a)n_(x) ^(c))^(½) and/or n_(y) ^(b)=(n_(y) ^(a) ^(c))^(½)(the square-root condition), will not have simultaneous suppression ofthe second, third and fourth-order reflection harmonics for normallyincident light. When the in-plane refractive indices deviate from thesquare-root condition, some combination of the second, third and/orforth-order reflection harmonics will develop reflectance while theother(s) will remain suppressed. The details of this departure fromsuppression depend on the optical thickness ratios of the polymers A, Band C, and on the way in which the square-root condition is violated.

[0061] Other embodiments of the present invention which satisfy FormulasII and/or III, and which have a unit cell arrangement ABCB, can havein-plane refractive indices that do not satisfy the square rootcondition, and also do not have the in-plane optical thickness ratiosf_(x) ^(a)=⅓, f_(x) ^(b)=⅙ and f_(x) ^(c)=⅓ and/or f_(y) ^(a)=⅓, f_(y)^(b)=⅙ and f_(y) ^(c)=⅓. In such instances, various combinations ofsecond, third and forth-order reflection harmonics can be suppressed,and control of the first-order reflection can be maintained as describedabove. Table II below shows several examples of this. TABLE II ExampleCases (ABCB Config.) in which Reflectivity Increases with IncidenceAngle Ny and/or Sign(N_(y) ^(a) − Sign(N_(y) ^(b) − Sign(N_(z) ^(a) −Sign(N_(z) ^(b) − Case Nx Nz Type N_(y) ^(b)) N_(y) ^(b)) N_(z) ^(b))N_(z) ^(b)) 7 A 1.78 1.50 B(+) (+) 0 B 1.67 1.50 B(+) (+) 0 C 1.50 1.50I 8 A 1.78 1.50 B(+) (+) (−) B 1.67 1.67 I (+) (−) C 1.50 1.78 B(−) 9 A1.78 1.50 B(+) (+) (−) B 1.67 1.67 I (+) 0 C 1.50 1.63 B(−) 10 A 1.781.63 B(+) (+) 0 B 1.67 1.67 I (+) (−) C 1.50 1.76 B(−)

[0062] The optical thickness ratios for polymeric materials A, B and Cmay have values that allow for suppression of differing combinations ofthe second, third and/or fourth-order reflection harmonics. Determiningrequired values of refractive indicies and f-ratios for polymers A, Band C, for suppression of combinations of two or more higher-orderharmonics, for isotropic materials at normal incidence, is explained inthe art. Such values may be determined by analytic techniques when boththe refractive index values and the f-ratios are considered unknowns(c.f. Muchmore R.,B., J. Opt. Soc. Am., 38 20,(1948), and Thelen, A.,J.Opt.Soc. Am. 53, 1266 (1963) ), or through numerical techniques whenthe refractive index values are fixed by realistic polymer choices.

[0063] For example, for certain polymer refractive index values of n_(x)^(b)=1.0255(n_(x) ^(a)n_(x) ^(c))^(½) and/or n_(y) ^(b)=1.0255(n_(y)^(a) _(y) ^(c))^(½) and n_(y) ^(a)1.772, n_(y) ^(c)=1.497 and/or n_(x)^(a)=1.772 and n_(x) ^(c)=1.497 if f_(x) ^(a)=0.200, f_(x) ^(b)=0.200and f_(x) ^(c)=0.400 and/or f_(y) ^(a)=0.200, f_(y) ^(b)=0.200 and f_(y)^(c)=0.400, then the second and third-order reflection harmonics will besuppressed. If, however, f_(x) ^(a)=(0.3846), f_(x) ^(b)=(0.1538) andf_(x) ^(c)=(0.3077) and/or f_(y) ^(a)= (0.3846), f_(y) ^(b)= (0.1538)and f_(y) ^(c)=(0.3077), then only the third and the forth-orderreflection harmonics will be suppressed. As discussed above,higher-order harmonic suppression will occur for normally incidentlight, and suppression (or lack thereof) of the higher-order harmonicsfor non-normally incident light, or in wavelength regions with highrefractive index dispersion, will differ for each case illustrated inTable II.

[0064] Cases 3 through 6 in Table I, and Cases 7 through 10 in Table II,illustrate specific examples of a more general result: Any polymericrepeating unit arranged in a multilayer stack consisting of polymericmaterials P₁, P₂, P₃, . . . P_(m), wherein the sign of the difference ofin-plane refractive indices between adjacent polymers P_(i) and P_(j+1)is opposite the sign of the difference in the z-axis refractive indicesbetween the same P_(i) and P_(i+1) for all polymeric interfaces, orwherein the sign of the difference in the z-axis refractive indicesbetween the same P_(i) and P_(i+1) is equal to 0 for all polymerinterfaces, will have increasing reflectance (of the first-orderharmonic) of unpolarized incident light with increasing angle ofincidence.

[0065] In the films and other optical bodies produced in accordance withthe present invention which are designed as IR reflectors, it isgenerally preferred that the polymeric layers of the optical repeatingunit show substantially no absorption in the visible part of thespectrum unless some color tint is desired. An infrared reflective filmproduced in accordance with the present invention preferably reflectsinfrared light over a wide range of wavelengths, and accordingly anoptical thickness variation is preferably introduced for the opticalrepeating unit along the thickness of the reflective film. In certainembodiments, sequences of optical repeat units with monotonicallyincreasing and decreasing optical thickness are desired. Methods fordesigning optical thickness gradients for the optical repeat units areset forth in U.S. Ser. No. ______ entitled “Optical Film with SharpenedBandedge”, filed under Attorney Docket no. 53545USA7A on Jan. 13^(th),1998 and incorporated herein by reference. The optical thickness of theoptical repeating unit may monotonically increase or decrease along theinfrared reflective film. Typically, an infrared reflective film inconnection with the present invention can be designed to have aninfrared reflective bandwidth of 200 nm to 1000 nm for a given opticalrepeat unit.

[0066] One skilled in the art will appreciate that a wide variety ofmaterials can be used to form mirrors or polarizers according to thepresent invention when these materials are processed under conditionsselected to yield the desired refractive index relationships. Thedesired refractive index relationships can be achieved in a variety ofways, including stretching during or after film formation (e.g., in thecase of organic polymers), extrusion (e.g., in the case of liquidcrystalline materials), or coating. It is preferred, however, that thetwo materials have similar rheological properties (e.g., meltviscosities) so that they can be co-extruded.

[0067] In general, appropriate combinations may be achieved by selectingfor each of layers A, B and C, a crystalline, semi-crystalline, orliquid crystalline material, or an amorphous polymer. Of course, it isto be understood that, in the polymer art, it is generally recognizedthat polymers are typically not entirely crystalline, and therefore inthe context of the present invention, crystalline orsemi-crystalline-polymers refer to those polymers that are not amorphousand includes any of those materials commonly referred to as crystalline,partially crystalline, or semi-crystalline.

[0068] Specific examples of suitable materials include polyethylenenaphthalate (PEN) and isomers thereof(e.g., 2,6-, 1,4-, 1,5-, 2,7-, and2,3-PEN), polyalkylene terephthalates (e.g., polyethylene terephthalate(PET), polybutylene terephthalate (PBT),poly-1,4-cyclohexanedimethyleneterephthalate) and copolymers of these,e.g., PETG, polyimides (e.g., polyacrylic imides), polyetherimides,polycarbonates (including copolymers such as the copolycarbonate of4,4′-thiodiphenol and bisphenol A in a 3:1 molar ratio, i.e., TDP),polymethacrylates(e.g., polyisobutyl methacrylate,polypropylmethacrylate,polyethylmethacrylate,andpolymethylmethacrylate),polyacrylates(e.g., polybutylacrylate andpolymethylacrylate),atactic polystyrene, syndiotactic polystyrene (sPS),syndiotactic poly-alpha-methyl styrene, syndiotacticpolydichlorostyrene, copolymers and blends of any of these polystyrenes,cellulose derivatives (e.g., ethyl cellulose, cellulose acetate,cellulose propionate, cellulose acetate butyrate, and cellulosenitrate), polyalkylene polymers (e.g., polyethylene, polypropylene,polybutylene, polyisobutylene, and poly(4-methyl)pentene),fluorinatedpolymers (e.g., perfluoroalkoxy resins,polytetrafluoroethylene,fluorinated ethylene-propylene copolymers,polyvinylidene fluoride, and polychlorotrifluoroethylene),chlorinatedpolymers (e.g., polyvinylidenechloride andpolyvinylchloride),polysulfones,polyethersulfones,polyacrylonitrile,polyamides, silicone resins, epoxyresins, polyvinylacetate,polyether-amides, ionomeric resins, elastomers(e.g.,. polybutadiene, polyisoprene, and neoprene), and polyurethanes.Also suitable are various copolymers, e.g., copolymers of PEN (e.g.,copolymers of 2,6-, 1,4-, 1,5-, 2,7-, and/or 2,3-naphthalenedicarboxylic acid, or esters thereof, with (a) terephthalic acid, oresters thereof; (b) isophthalic acid, or esters thereof; (c) phthalicacid, or esters thereof; (d) alkane glycols; (e) cycloalkane glycols(e.g., cyclohexane dimethanol diol); (f) alkane dicarboxylic acids;and/or (g) cycloalkane dicarboxylic acids (e.g., cyclohexanedicarboxylic acid)), copolymers of polyalkylene terephthalates (e.g.,copolymers of terephthalic acid, or esters thereof, with (a) naphthalenedicarboxylic acid, or esters thereof; (b) isophthalic acid, or estersthereof; (c) phthalic acid, or esters thereof; (d) alkane glycols; (e)cycloalkane glycols (e.g., cyclohexane dimethane diol); (f) alkanedicarboxylic acids; and/or (g) cycloalkane dicarboxylic acids (e.g.,cyclohexane dicarboxylic acid)), and styrene copolymers (e.g.,styrene-butadienecopolymers and styrene-acrylonitrile copolymers),4,4′-bibenzoic acid and ethylene glycol. In addition, each individuallayer may include blends of two or more of the above-described polymersor copolymers (e.g., blends of syndiotactic polystyrene (sPS) andatactic polystyrene).

[0069] A particularly preferred birefringent polymeric material for usein one or more layers of the films and other optical devices produced inaccordance with the present invention is a crystalline orsemi-crystalline polyethylenenaphthalate (PEN), inclusive of its isomers(e.g. 2,6-; 1,4-; 1,5-; 2,7; and 2,3-PEN). Particularly preferredisotropic polymeric materials for use in the present invention includepolyacrylates and in particular, polymethylmethacrylate (PMMA). Oneskilled in the art will appreciate that each of the polymeric layers A,B and C may be composed of blends of two or more polymeric materials toobtain desired properties for a specific layer.

[0070] Preferred optical repeating units in connection with the presentinvention are optical repeating units that have polymeric layers A, Band C arranged in an order AABCCB. Especially preferred are thosewherein layers A, B and C comprise the various sets of polymers asdescribed in the following paragraphs.

[0071] The following material embodiments for the various optical casesare non-exclusive examples based on modeled data, since many suchsystems exist. For example, PBN (poly butylene naphthalate) or even anon-polyester can be used for the material of highest in-plane index.Index of refraction values are approximate and assume a wavelength of632.8 nm. Copolymers of PEN with other polyesters are listed ascoPEN×/100−x, where x is the approximate percentage of NDC (naphthalenedicarboxylate) content and can vary by more than +/−20%. Unlessotherwise noted, the coPENs are considered essentially unoriented byattention to the process conditions and by suitable selection of thecomponents in the non-NDC fraction (100−x).

[0072] The example material systems listed in TABLE III below refer tothe case examples illustrated in TABLE I and TABLE II and FIGS. 2-7.Some of the example material cases involve very similar polymers, butachieve different refractive index results through differing processconditions. TABLE III Case No.* Materials Approx N_(y) and N_(x) ApproxN_(z) Case 1 A = PEN 1.74 1.48 B = coPEN 60/40 1.61 1.61 C = PMMA 1.491.49 - OR - A = PEN 1.74 1.48 B = coPEN 70/30 1.62 1.62 C = ECDEL ™ 1.521.52 (here ECDEL ™ is one of many aliphatic polyesters with this lowvalue of isotropic index of refraction) - OR - A = PEN 1.74 1.48 B = TDP1.63 1.63 C = ECDEL ™ 1.52 1.52 - OR - A = PET 1.65 1.49 B = PC 1.571.57 C = PMMA 1.49 1.49 (Here PC can be a standard bis-phenol A type.) -OR - A = PET 1.65 1.49 B = PETG 1.57 1.57 C = PMMA 1.49 1.49 Case 7 A =PEN 1.74 1.48 B = PET 1.65 1.49 C = PMMA 1.49 1.49 Case 3 A = PEN 1.741.48 B = PBT 1.63 1.47 C = PMMA 1.49 1.49 Case 4 A = PET 1.65 1.49 B =coPEN 50/50 1.60 1.60 C = sPS 1.55 1.63 Case 5 A = PEN 1.74 1.50 B =coPEN 1.63 1.63 C = sPS 1.55 1.63

[0073] This invention also anticipates the formation of a multicomponentpolarizing film for longer wavelengths (e.g., the near IR) which wouldalso be substantially transparent in the visible region of the spectrum.In TABLES I and II, this is the case of one in-plane index set meetingthe required conditions while the orthogonal set of in-plane indices aresubstantially matched across the IR spectrum of interest. Examples ofsuch films may be constructed using the multiple drawing step processesin U.S. Ser. No. ______ entitled “An Optical Film and Process forManufacture Thereof” and filed under attorney Docket No. 53546USA5A onJan. 13, 1998. One particular combination for materials A and C mayinclude PEN and a copolymer comprising 10% of PEN type subunits and 90%of PET type subunits, i.e., an orienting and crystallizable 10/90 co-PEN(as is obtained from a coextruded transesterified blend of 10 weight %PEN and 90 weight % PET). The choice of material B could be anintermediate copolymer of these, e.g., an orienting and crystallizable70/30 co-PEN. In general, a variety of IR polarizers could beconstructed by a variety of material combinations with this method.

[0074] The films and other optical devices made in accordance with theinvention may also include one or more anti-reflective layers orcoatings, such as, for example, conventional vacuum coated dielectricmetal oxide or metal/metal oxide optical films, silica sol gel coatings,and coated or coextruded antireflective layers such as those derivedfrom low index fluoropolymers such as THV, an extrudable fluoropolymeravailable from 3M Company (St. Paul, Minn.). Such layers or coatings,which may or may not be polarization sensitive, serve to increasetransmission and to reduce reflective glare, and may be imparted to thefilms and optical devices of the present invention through appropriatesurface treatment, such as coating or sputter etching.

[0075] Both visible and near IR dyes and pigments are contemplated foruse in the films and other optical bodies of the present invention, andinclude, for example, optical brighteners such as dyes that absorb inthe UV and fluoresce in the visible region of the color spectrum. Otheradditional layers that may be added to alter the appearance of theoptical film include, for example, opacifying (black) layers, diffusinglayers, holographic images or holographic diffusers, and metal layers.Each of these may be applied directly to one or both surfaces of theoptical film, or may be a component of a second film or foilconstruction that is laminated to the optical film. Alternately, somecomponents such as opacifying or diffusing agents, or colored pigments,may be included in an adhesive layer which is used to laminate theoptical film to another surface.

[0076] Suitable methods for making reflective multilayer films of thetype toward which the present invention is directed are described, forexample, in U.S. Ser. No. ______ entitled “Process for Making MultilayerOptical Film” filed under attorney docket no. 51932USA8A on Jan. 13,1998, and which is hereby incorporated by reference. However, some ofthe considerations involved in these methods are discussed brieflybelow.

[0077] It is preferred that the polymers have compatible rheologies tofacilitate coextrusion. That is, since the use of coextrusion techniquesis preferred in making the films and other optical bodies of the presentinvention, the melt viscosities of the polymers are preferablyreasonably matched to prevent layer instability or non-uniformity. Thepolymers used also preferably have sufficient interfacial adhesion sothat the resulting films will not delaminate.

[0078] The multilayer reflective films of the present invention can bereadily manufactured in a cost effective way, and can be formed andshaped into a variety of useful configurations after coextrusion.Multilayer infrared reflective films in accordance with the presentinvention are most advantageously prepared by employing multilayeredcoextrusion devices such as those described in U.S. Pat. Nos. 3,773,882and 3,884,606, the disclosures of which are incorporated herein byreference. Such devices provide a method for preparing multilayered,simultaneously extruded thermoplastic materials, each of which are of asubstantially uniform layer thickness. Preferably, a series of layermultiplying means as are described in U.S. Pat. No. 3,759,647, thedisclosure of which is incorporated herein by reference, are employed.

[0079] The feedblock of the coextrusion device receives streams of thediverse thermoplastic polymeric materials from a source such as a heatplastifying extruder. The streams of resinous materials are passed to amechanical manipulating section within the feedblock. This sectionserves to rearrange the original streams into a multilayered streamhaving the number of layers desired in the final body. Optionally, thismultilayered stream may be subsequently passed through a series of layermultiplying means in order to further increase the number of layers inthe final body.

[0080] The multilayered stream is then passed into an extrusion diewhich is so constructed and arranged that stream-lined flow ismaintained therein. Such extrusion devices are described in U.S. Pat.No. 3,557,265, the disclosure of which is incorporated by reference. Theresultant product is extruded to form a multilayered body in which eachlayer is generally parallel to the major surface of adjacent layers.

[0081] The configuration of the extrusion die may vary and can be suchas to reduce the thickness and dimensions of each of the layers. Theprecise degree of reduction in thickness of the layers delivered fromthe mechanical orienting section, the configuration of the die, and theamount of mechanical working of the body after extrusion are all factorswhich affect the thickness of the individual layers in the final body.

[0082] The number of layers in the reflective films and other opticaldevices made in accordance with the present invention can be selected toachieve the desired optical properties using the minimum number oflayers for reasons of film thickness, flexibility and economy. In thecase of both infrared reflective polarizers and infrared reflectivemirrors, the number of layers is preferably less than about 10,000, morepreferably less than about 5,000, and most preferably, less than about2,000.

[0083] The desired relationship between refractive indices of polymericlayers A, B and C as desired in this invention can be achieved byselection of appropriate processing conditions. In the case of organicpolymers which can be oriented by stretching, the multilayer films aregenerally prepared by co-extruding the individual polymers to form amultilayer film (e.g., as set out above) and then orienting the film bystretching at a selected temperature, optionally followed byheat-setting at a selected temperature. Alternatively, the extrusion andorientation steps may be performed simultaneously. By the orientation,the desired extent of birefringence (negative or positive) is set inthose polymeric layers that comprise a polymer that can exhibit abirefringence. Positive birefringence is obtained with polymers thatshow a negative optical stress coefficient, i.e., polymers for which thein-plane indices will decrease with orientation whereas negativebirefringence is obtained with polymers having a positive optical stresscoefficient.

[0084] In the case of polarizers, the film is typically stretchedsubstantially in one direction (uniaxial orientation), while in the caseof mirrors, the film can be stretched substantially in two directions(biaxial orientation). However, with the proper selection of conditions,polarizing films can be made through biaxial orientation. Such films maybe made, for example, by stretching the film under such conditions thatparticular layers are selectively oriented and other layers are not.Suitable methods for producing biaxial polarizers in accordance with thepresent invention are described, for example, in U.S. Ser, No. ______entitled “An Optical Film and Process for Manufacture Thereof” and filedunder attorney docket number 53546USA5A on Jan. 13, 1998.

[0085] In the case of mirrors, the stretching may be asymmetric tointroduce specially desired features, but is preferably symmetric.Reflective mirrors may also be obtained in accordance with the presentinvention by laminating together two infrared reflective films that haveeach been uniaxially oriented in such a way that their axes oforientation are rotated 90° to one another.

[0086] The film may be allowed to dimensionally relax in thecross-stretch direction from the natural reduction in cross-stretch(equal to the square root of the stretch ratio), or may be constrainedso that there is no substantial change in cross-stretch dimensions. Thefilm may be stretched in the machine direction, as with a lengthorienter, and/or in the transverse or width direction using a tenter.

[0087] The pre-stretch temperature, stretch temperature, stretch rate,stretch ratio, heat set temperature, heat set time, heat set relaxation,and cross-stretch relaxation are selected to yield a multilayer devicehaving the desired refractive index relationship. These variables areinter-dependent; thus, for example, a relatively low stretch rate couldbe used if coupled, for example, with a relatively low stretchtemperature. One skilled in the art will appreciate that variouscombinations of these variables may be selected to achieve a desiredmultilayer device. In general, however, a stretch ratio in the rangefrom about 1:2 to about 1:10 (more preferably about 1:3 to about 1:7) inthe stretch direction and from about 1:0.2 to about 1:10 (morepreferably from about 1:0.2 to about 1:7) orthogonal to the stretchdirection is preferred.

[0088] Suitable multilayer films may also be prepared using techniquessuch as spin coating (e.g., as described in Boese et al., J. Polym.Sci.: Part B, 30:1321 (1992) for birefringentpolyimides), and vacuumdeposition (e.g., as described by Zang et. al., Appl. Phys. Letters,59:823 (1991) for crystalline organic compounds). The latter techniqueis particularly useful for certain combinations of crystalline organiccompounds and inorganic materials.

[0089] Orientation of the extruded film can be accomplished bystretching individual sheets of the material in heated air. Foreconomical production, stretching may be accomplished on a continuousbasis in a standard length orienter, tenter oven, or both. Economies ofscale and line speeds of standard polymer film production may beachieved, thereby reducing manufacturing costs below levels associatedwith commercially available absorptive polarizers.

[0090] Lamination of two or more reflective films together isadvantageous to improve reflectivity or to broaden the bandwidth, or toform a mirror from two polarizers as described above. Amorphouscopolyesters, such VITEL™ 3000 and 3300 which are commercially availablefrom the Goodyear Tire and Rubber Co. of Akron, Ohio, are useful aslaminating materials. The choice of laminating material is broad, withdegree of adhesion, optical clarity and exclusion of air being theprimary guiding principles.

[0091] It may be desirable to add to one or more of the layers one ormore inorganic or organic adjuvants such as an antioxidant, extrusionaid, heat stabilizer, ultraviolet ray absorber, nucleator, surfaceprojection forming agent, and the like in normal quantities so long asthe addition does not substantially interfere with the performance ofthe present invention.

[0092] A practical situation that can arise in the selection of a glueor “tie” layer is that it is common for elastomers, polyolefins, andother polymers which are good candidate tie layer materials to beisotropic and have the lowest refractive index of any of the repeat cellcomponents (often they are in the 1.47-1.52 range). Issues of materialcost, optical haze, absorptive color, or overall mechanical propertiesmay preclude using the thickness of the tie layer required for a twocomponent system consisting of the low index tie layer with a high indexmaterial in a 2 component repeat cell. A solution to this problem is todesign the 3 component repeat cell in a pattern A/C/B/C wheren^(a)>n^(b)>n^(c) in at least one in-plane direction and wherein C isthe aforementioned tie layer having the lowest index (it is alsopossible that Nb=Nc). For the aforementioned reasons it may be desirableto make the thickness of the C layer as small as possible to minimizethe cost and optical haze/absorptive color problems.

[0093] If a film of the present invention is designed to reflect lightin the infrared region, it may be preferable to further design the filmto avoid a change in perceived color as the viewing angle or angle ofincidence of light changes, for example, from normal incidence tonon-normal incidence, while maintaining the ability to provide infraredblocking properties over as much of the infrared region of the spectrumas possible. For typical dielectric multilayer films, if the reflectingband is positioned to reflect the maximum amount of solar radiation atnormal angles of incidence while remaining clear in the visible regionof the spectrum, the short wavelength bandedge is positioned at or nearthe visible wavelength cutoff, i.e. at about 700 nm. The reflecting bandmoves to shorter wavelengths at non-normal angles of incidence, however,so that while the film appears clear at normal angles, it will becolored at non-normal angles.

[0094] For some applications, it is desirable that the film appear clearat all angles of light incidence or viewing angles, and to accomplishthis, the reflecting band must be positioned at longer wavelengthswithin the infrared so that the short wavelength bandedge does not shiftinto the visible region of the spectrum even at maximum use angles. Thiscan be accomplished by designing an infrared reflecting film of thepresent invention so that the film has a reflecting band positioned toreflect infrared radiation of at least one polarization at an incidentangle normal to the film, where the reflecting band has a shortwavelength bandedge λ_(a0) and long wavelength bandedge λ_(b0) at anormal incident angle, and a short wavelength bandedge λ_(aθ) and longwavelength bandedge λ_(bθ) at a maximum usage angle θ, wherein λ_(aθ) isless than λ_(a0) and λ_(a0) is selectively positioned at a wavelengthgreater than about 700 nm. At least one component can then be providedas part of the film or in addition to the film which at least partiallyabsorbs or reflects radiation in the wavelength region between λ_(aθ)and λ_(a0) at a normal angle of incidence.

[0095] The component functions to either absorb or reflect the infraredwavelengths that are not reflected by the film at normal angles becauseof the positioning of the reflective band of the multilayer film athigher wavelengths in order to minimize perceived color changes atnon-normal incidence. Depending on the placement of the gap fillercomponent relative to the film, the component may not function atnon-normal angles because the reflective band gap due to the reflectionof the multilayer film shifts to lower wavelengths, preferablycoinciding with the wavelength region of the absorption or reflection ofthe gap filler component. Placement of the reflective band within theinfrared and components useful for filling the resulting band gap thatoccurs at normal angles are more fully described in U.S. patentapplication Ser. No. ______ entitled “Multilayer Infrared ReflectingOptical Body,” filed by applicants on Feb. 13, 1998 under AttorneyDocket No. 52994USA7A, the contents of which are herein incorporated byreference.

[0096] Suitable gap filler components include an infrared absorbing dyeor pigment, an infrared absorbing glass, a trailing segment, a pluralityof isotropic layers, or combinations thereof The gap filler componentmay be a part of the film, for example, as a trailing segment or aplurality of isotropic layers coextruded with the film layers or as adye or pigment incorporated into one or more of the film layers.

[0097] Alternatively, the gap filler component may be a discrete part ofthe optical body of the present invention, i.e., separate from the film,that is attached, for example, laminated thereto. Examples of thisembodiment include a dye or pigment as a separate layer adhered to thefilm. The description of the gap filler as a part of the film andseparate from the film is merely exemplary. The gap filler componentdisclosed herein may be either be a part of the film or may be separatefrom the film depending on the characteristics of the component itselfand the film with which it is being combined.

[0098] The film and the gap filler components are preferably combinedsuch that the film is placed on a surface nearest the sun as practicalbecause it is more efficient to reflect solar energy than to absorb it.In other words, where possible, it is preferable that the sun's raysfirst encounter the film and then secondarily encounter the gap fillercomponent. In a multiple pane or two-ply windshield, the most preferableplacement for the film is the exterior nearest the sun, the nextpreferably position is between the panes or plies. The film may beplaced on the interior surface but this allows absorption of solar lightby the glass before the light reaches the film and absorption of part ofthe light reflected from the film. This embodiment may be preferablewhen considered from a UV protection standpoint, since it may bepreferable to position the film away from the sun, allowing componentswhich are less sensitive to UV to absorb this part of the light.

[0099] Examples of suitable infrared absorbing dyes include cyanine dyesas described, for example, in U.S. Pat. No. 4,973,572, herebyincorporated by reference, as well as bridged cyanine dyes andtrinuclear cyanine dyes as described, for example, in U.S. Pat. No.5,034,303, hereby incorporated by reference, merocyanine dyes asdescribed, for example, in U.S. Pat. No. 4,950,640, hereby incorporatedby reference, carbocyanine dyes (for example,3,3′-diethyloxatricarbocyanine iodide, 1,1′, 3,3,3′,3′-hexamethylindotricarbocyanine perchlorate,1,1′,3,3,3′,3′-hexamethylindotricarbocyanine iodide,3,3′-diethylthiatricarbocyanine iodide, 3,3′-diethylthiatricarbocyanineperchlorate, 1,1′,3,3,3′, 3′-hexamethyl-4,4′,5,5′-dibenzo-2,2′-indotricarbocyanine perchlorate, all of which arecommercially available from Kodak, Rochester, N.Y.), and phthalocyaninedyes as described, for example, in U.S. Pat. No. 4,788,128, herebyincorporated by reference; naphthaline dyes; metal complex dyes, forexample, metal dithiolate dyes (for example, nickel dithiolate dyes and,for example, bis[4-dimethylaminodithiobenzil] nickel, bis[dithiobenzil]nickel, bis[1,2-bis(n-butylthio)ethene-1,2-dithiol]nickel bis[4,4′-dimethoxydithiobenzil] nickel, bis[dithiobenzil] platinum,bis[dithioacetyl] nickel) and metal dithiolene dyes (for example,nickeldithiolene dyes as described, for example, in U.S. Pat. No.5,036,040, hereby incorporated by reference); polymethine dyes such asbis(chalcogenopyrylo)polymethine dyes as described, for example, in U.S.Pat. No. 4,948,777, hereby incorporated by reference, bis(aminoaryl)polymethine dyes as described, for example, in U.S. Pat. No. 4,950,639,hereby incorporated by reference, indene-bridged polymethine dyes asdescribed, for example, in U.S. Pat. No. 5,019,480, hereby incorporatedby reference, and tetraaryl polymethine dyes; diphenylmethane dyes;triphenylmethane dyes; quinone dyes; azo dyes; ferrous complexes asdescribed, for example, in U.S. Pat. No. 4,912,083, hereby incorporatedby reference; squarylium dyes as described, for example, in U.S. Pat.No. 4,942,141, hereby incorporated by reference;chalcogenopyryloarylidene dyes as described, for example, in U.S. Pat.No. 4,948,776, hereby incorporated by reference; oxoindolizine dyes asdescribed, for example, in U.S. Pat. No. 4,948,778, hereby incorporatedby reference; anthraquinone and naphthoquinone derived dyes asdescribed, for example, in U.S. Pat. No. 4,952,552, hereby incorporatedby reference; pyrrocoline dyes as described, for example, in U.S. Pat.No. 5,196,393, hereby incorporated by reference; oxonol dyes asdescribed, for example, in U.S. Pat. No. 5,035,977, hereby incorporatedby reference; squaraine dyes such as chromylium squaraine dyes,thiopyrylium squaraine dyes as described, for example, in U.S. Pat. No.5,019,549, hereby incorporated by reference, and thiochromyliumsquaraine dyes; polyisothianaphthene dyes; indoaniline and azomethinedyes as described, for example, in U.S. Pat. No. 5,193,737, herebyincorporated by reference; indoaniline methide dyes; tetraarylaminiumradical cation dyes and metallized quinoline indoaniline dyes.Squarylium dyes or squaraines are also described, for example, in U.S.Pat. No. 4,942,141 and U.S. Pat. No. 5,019,549, both of which are herebyincorporated by reference.

[0100] Commercially available phthalocyanine dyes include, for example,those available from Zeneca Corporation, Blackley, Manchester, Englandunder the trade designation “Projet Series” for example, “Projet 830NP”,“Projet 860 NP” and “Projet 900NP”.

[0101] Commercially available metal complex dyes include those availablefrom C. C. Scientific Products, Ft. Worth, Tex. 76120, for example,bis[4-dimethylaminodithiobenzil] nickel.

[0102] Additional suitable dyes include those described in JurgenFabian's article entitled “Near Infrared Absorbing Dyes” Chem Rev, 1992,1197-1226 and “The Sigma Aldrich Handbook of Stains, Dyes andIndicators” by Floyd J. Green, Aldrich Chemical Company, Inc.,Milwaukee, Wis. ISBN 0-941633-22-5,1991, both of which are herebyincorporated by reference. Useful near infrared absorbing dyes includethose from Epolin, Inc., Newark, N.J., for example, having the tradedesignations: Epolight III-57, Epolight III-117, Epolight V-79, EpolightV-138, Epolight V-129, Epolight V-99, Epolight V-130, Epolight V-149,Epolight IV-66, Epolight IV-62A, and Epolight III-189.

[0103] Suitable infrared absorbing pigments include cyanines, metaloxides and squaraines. Suitable pigments include those described in U.S.Pat. No. 5,215,838, incorporated herein by reference, such as metalphthalocyanines, for example, vanadyl phthalocyanine, chloroindiumphthalocyanine, titanyl phthalocyanine, chloroaluminum phthalocyanine,copper phthalocyanine, magnesium phthalocyanine, and the like;squaraines, such as hydroxy squaraine, and the like; as well as mixturesthereof. Exemplary copper pthalocyanine pigments include the pigmentcommercially available from BASF under the trade designation “6912”.Other exemplary infrared pigments include the metal oxide pigmentcommercially available from Heubach Langelsheim under the tradedesignation “Heucodor”.

[0104] Dyes or pigments useful in the present invention may benarrow-band absorbing, absorbing in the region of the spectrum notcovered because of the position of the short wavelength bandedge of theoptical body, for example, 700 to 850 nm, or may be broad band,absorbing over substantially all or all of the infrared region.

[0105] The dye or pigment can be applied to either surface of the film,in a layer of glass or polymer, such as polycarbonate or acrylic,laminated to the film, or be present in at least one of the polymerlayers of the film. From a solar energy standpoint, the dye ispreferably on the innermost surface of the film (i.e. toward the roominterior and away from the sun) so that when the sun is a high angle,the film reflective band shifts to lower wavelengths, essentiallycoinciding with the wavelength region of the dye. This is preferredbecause reflecting solar energy away from the building is preferred toabsorbing it.

[0106] The amount of dye or pigment used in the optical body of thepresent invention varies depending on the type of dye or pigment and/orthe end use application. Typically, when applied to the surface of thefilm, the dye or pigment is present on the surface at a concentrationand coating thickness suitable to accomplish the desired infraredabsorption and visible appearance. Typically, if the dye or pigment iswithin an additional layer or within the multilayer optical body, theconcentration ranges from about 0.05 to about 0.5 weight %, based on thetotal weight of the optical body. In addition, when a pigment is used, asmall particle size typically is needed, for example, less than thewavelength of light. If the dyes are non-polar solvent soluble, the dyescan be coated or mixed in with solid plastic pellets and extruded if thedyes can withstand the heat of mixing and extrusion.

[0107] Examples of suitable infrared absorbing glasses include clearglass having a thickness generally ranging from about 3 to about 6 mm,such as architectural or automotive glass; blue glass; or green glasswhich selectively absorb in the near infrared, i.e., about 700 to 1800nm.

[0108] In the embodiments where blue or green glass is used, it ispreferable that the film of the present invention is located on thesurface of the glass closest to the sun so that the film can reflectaway the 850-1250 nm wavelengths, allowing some of the infrared which isnot reflected to be absorbed by the glass. If it is not practical toplace the film on the exterior surface of a glass layer, for example, onthe exterior of a window of a building, it may be useful to place thefilm between panes of glass, rather than on the surface closest to theinterior, in the case of multiple pane windows, in order to minimizeabsorption. Preferably, the exterior layer (closest to the sun) hasminimal infrared absorbing properties so that the film is able toreflect light in the infrared region before this light reaches theinterior infrared absorbing glass. In this embodiment, the glasstemperature would be lower and less heat would enter the room due tore-radiation of absorbed light. Additionally, the glass and/or filmwould be cooler which would reduce cracking of the glass due to thermalstress, a common problem with heavily absorbing materials.

[0109] Infrared absorbing glass is available commercially from companiesincluding Pittsburgh Plate Glass (PPG), Guardian, Pilkington-LibbeyOwens Ford, Toledo, Ohio.

[0110] Generally a sharp band edge is desired in optical interferencefilms such as the infrared reflective films described herein. Sharp bandedges can be obtained from proper design of the layer thickness gradientthroughout the multilayer optical stack, as described in U.S. Ser. No.______ entitled “Optical Film with Sharpened Bandedge” filed byapplicants on Jan. 13, 1998 under Attorney Docket No. 53545USA7A.Instead, a reflective film of the present invention can be designed toinclude a trailing segment to partially reflect infrared wavelengths inthe gap region without producing strong color in the visible spectrum atnon-normal angles. A trailing segment can be provided as a multilayerinterference film have layer thicknesses and refractive indices suchthat the reflectance in the gap region is relatively weak, for example,50% and which may decrease so that transfer from high reflectance to lowreflectance of the multilayer film is gradual. For example, a layergradient may provide a sharp bandedge above, for example, the 50%reflectance point and a trailing segment could be provided by additionallayers. For example, instead of providing a sharp edge, the last 30layers of a 200 layer stack could be of appropriate optical thicknessthat their first order reflection occurs in the range of about 800-850nm, the intensity of which increase from about 90% reflection at 850 nmto about 25% at 800 nm. The other 170 layers could provide, for example,about 90% reflection from about 850-1150 nm. Achieving the trailingsegment can be done in a number of ways, for example, by controlling thevolumetric feed of the individual layers. The trailing segment may beextruded with the multilayer film of the present invention or laminatedthereto.

[0111] Possible advantages of a trailing segment is that instead of anabrupt transition from no color to maximum color, the trailing segmentprovides a “softer” transition which may be more aestheticallyacceptable and easier to control from a process standpoint.

[0112] An isotropic multilayer film may also be used to cover at least aportion of the wavelength gap. Isotropic layers lose p-pol reflectionintensity at oblique angles. Accordingly, at oblique angles, the z-indexmatched reflectance band would shift into the gap and the reflectancefrom the isotropic layers would shift to the visible but also decreasein p-pol intensity. S-pol would be masked or partially masked by theair/optical body surface which would increase its reflectance at obliqueangles. Exemplary isotropic polymers include but are not limited toisotropic coPEN, PMMA, polycarbonates, styrene acrylonitriles, PETG,PCTG, styrenics, polyurethanes,polyolefins, and fluoropolymers. Theisotropic film may be coextruded with a film of the present invention orlaminated to a film of the present invention.

[0113] Preferably, gap filler component is situated such that light hitsthe film of the present invention before it hits the gap fillercomponent so that, then when the sun is at normal incidence, the gapfiller absorbs light in the region of the gap. However, when the sun isat high angles, the film will shift to some of the same wavelengths asthe gap filler component and serve to reflect at least some of the lightin the region of the gap.

[0114] Gap filler components may be used in combination with the film ofthe present invention, for example, when each gap filler component onlyabsorbs or reflects in a portion of the gap to be filled. In addition,shifting the bandedge and, thus, creating the gap, also serves to createanother, or second, gap in the infrared region at longer wavelengths offangle. Therefore, it may be preferable to also include another componentwhich fills this second gap region off angle. Suitable gap fillercomponents to fill this second gap include dyes, pigments, glasses,metals and multilayer films which absorb or reflect in the longerwavelengths of the infrared region.

[0115] The preceding description of the present invention is merelyillustrative, and is not intended to be limiting. Therefore, the scopeof the present invention should be construed solely by reference to theappended claims.

What is claimed is:
 1. A reflective film that reflects light in theinfrared region of the spectrum while transmitting light in the visibleregion of the spectrum comprising an optical repeating unit comprisingpolymeric layers A, B and C arranged in an order ABC, said polymericlayer A having refractive indices n_(x) ^(a) and n_(y) ^(a) alongin-plane axes x and y respectively, said polymeric layer B havingrefractive indices n_(x) ^(b) and n_(y) ^(b) along in-plane axes x and yrespectively, said polymeric layer C having refractive indices n_(x)^(c) and n_(y) ^(c) along in-plane axes x and y respectively, polymericlayers A, B and C having a refractive index n_(z) ^(a), n_(z) ^(b) andn_(z) ^(c) respectively along a transverse axis z perpendicular to thein-plane axes, wherein n_(x) ^(b) is intermediate n_(x) ^(a) and n_(x)^(c), with n_(x) ^(a) being larger than n_(x) ^(c) and/or n_(y) ^(b) isintermediate to n_(y) ^(a) and _(y) ^(c) with n_(y) ^(a) being largerthan n_(y) ^(c) and wherein at least one of the differences n_(z)^(a)−n_(z) ^(b) and n_(z) ^(b)−n_(z) ^(c) is less than 0 or both saiddifferences substantially equal
 0. 2. A reflective film according toclaim I wherein said optical repeating unit comprises said polymericlayers A, B and C in a pattern ABCB.
 3. A reflective film according toclaim 2 wherein said polymeric layer A has an optical thickness ratiof_(x) ^(a)=⅓ along said in-plane axis x, said polymeric layer B has anoptical thickness ratio f_(x) ^(b)=⅙ along said in-plane axis x, saidpolymeric layer C has an optical thickness ratio f_(x) ^(c)=⅓ along saidin-plane axis x, and n_(x) ^(b)=(n_(x) ^(a)n_(x) ^(c))^(½) and/orwherein said polymeric layer A has an optical thickness ratio f_(y)^(a)=⅓ along said in-plane axis y, said polymeric layer B has an opticalthickness ratio f_(y) ^(b)=⅙ along said in-plane axis y, said polymericlayer C has an optical thickness ratio f_(y) ^(c)=⅓ along said in-planeaxis y, and n_(y) ^(b)=(n_(y) ^(a)n_(y) ^(c))^(½).
 4. A reflective filmaccording to any of the previous claims wherein when said differencesare of opposite sign, the difference that is less than 0 having thelargest absolute value or the absolute value of both differences beingsubstantially equal.
 5. A reflective film according to any of claims 1to 3 wherein both said differences are less than or equal to −0.05 orone of said differences equals 0 and the other difference is less thanor equal to −0.05.
 6. A reflective film according to any of claims 1 to3 wherein n_(z) ^(b) differs by not more than 0.03 from n_(z) ^(a) andn_(z) ^(c).
 7. A reflective film according to any of the previous claimswherein said optical repeating unit has a monotonically varying opticalthickness along at least part of the thickness of said reflective film.8. A reflective film according to any of the previous claims whereinsaid reflective film comprises a multilayer film M1 and a multilayerfilm M2 each having first order reflections in the infrared part of thespectrum, multilayer film M1 comprising an optical repeating unit R1,multilayer film M2 comprising an optical repeating unit R2, said opticalrepeating unit R1 monotonically varying in optical thickness along thethickness of said multilayer film M1 and said optical repeating unit R2being of substantially constant optical thickness along the thickness ofmultilayer film M2 and the optical thickness of optical repeating unitR2 being less than or equal to the minimum optical thickness of opticalrepeating unit R1 along the thickness of multilayer film M1 or theoptical thickness of optical repeating unit R2 being equal to or greaterthan the maximum optical thickness of optical repeating unit R1 alongthe thickness of multilayer film M1 or said optical repeating unit R2monotonically varying in optical thickness along the thickness of saidmultilayer film M2 opposite to said monotonically optical thicknessvariation of optical repeating unit R1 and the minimum optical thicknessof optical repeating unit R2 along the thickness of multilayer film M2being substantially equal to the minimum optical thickness of opticalrepeating unit R1 along the thickness of multilayer film M1 or themaximum optical thickness of optical repeating unit R2 along thethickness of multilayer film M2 being substantially equal to the maximumoptical thickness of optical repeating unit R1 along the thickness ofmultilayer film M1.
 9. A reflective film according to claim 8 whereinsaid optical repeating unit R1 is an optical repeating unit as definedin any of claims 1 to 6 .
 10. A reflective film according to claim 9wherein optical repeating unit R2 consists of a first and secondpolymeric layer.
 11. A reflective film according to claim 10 whereinsaid first and second polymeric layer have an index of refraction n_(z)¹ and n_(z) ² respectively along the z-axis and the difference betweenn_(z) ¹ and n_(z) ² is not more than 0.03.
 12. A reflective filmaccording to any of the previous claims wherein said reflective filmreflects infrared light over a bandwidth of 500 to 1000 nm.
 13. Areflective film according to any of the previous claims wherein saidreflective film further comprises a skin layer on one or both surfaces.14. An infrared reflecting mirror comprising a reflective film asdefined in any of claims 1 to 12 .
 15. An infrared reflecting mirroraccording to claim 14 comprising a first reflective film as defined inany of claims 1 to 12 and a second reflective film as defined in any ofclaims 1 to 12 , said first and second reflective films being unaxiallyoriented and arranged such that their axes of orientation are 90° C.turned to one another.
 16. A material comprising on a support areflective film as defined in any of claims 1 to 13 .
 17. A materialaccording to claim 16 wherein said support is transparent to visiblelight.
 18. A material according to claim 17 wherein said support isglass or a plastic film.
 19. Method of reflecting infrared lightcomprising the steps of providing a reflective film as defined in any ofclaims 1 to 13 and allowing at least a portion of incident infraredlight to reflect from said reflective film.
 20. Method according toclaim 19 wherein at least 50% of incident infrared light having awavelength between 750 nm and 1600 nm is reflected.
 21. Method ofreflecting infrared light comprising the steps of providing an infraredmirror as defined in any of claims 14 or 15 and allowing at least aportion of incident infrared light to reflect from said reflective film.22. Method according to claim 21 wherein at least 50% of incidentinfrared light having a wavelength between 750 nm and 1600 nm isreflected.
 23. Use of an infrared reflective film as defined in any ofclaims 1 to 13 to reduce incidence of infrared light in a room.
 24. Anoptical body, comprising: a first polymeric layer A; a second polymericlayer B; and a third polymeric layer C; wherein A, B, and C are arrangedin the sequence ABC, where B is contiguous to A and C; wherein A hasrefractive indices n_(x) ^(A), n_(y) ^(A), and n_(z) ^(A) for lightpolarized along mutually orthogonal axes x, y, and z, respectively, Bhas refractive indices n_(x) ^(B), n_(y) ^(B), and n_(z) ^(B) for lightpolarized along axes x, y and z, respectively, and C has refractiveindices n_(x) ^(C), n_(y) ^(C) and n_(z) ^(C) for light polarized alongaxes x, y, and z, respectively; wherein axis z is orthogonal to layer B;and wherein the indices of refraction of layers A, B, and C are inaccordance with Formula I and at least one of Formulas II and III: n_(z)^(C)≧n_(z) ^(B)≧n_(z) ^(A)  (Formula I) n_(x) ^(A)>n_(x) ^(B)>n_(x)^(C)  (Formula II) n_(y) ^(A)>n_(y) ^(B)>n_(y) ^(C)  (Formula III). 25.The optical body of claim 24 , wherein said optical body comprises aplurality of repeating units having the layer sequence ABC.
 26. Theoptical body of claim 24 , wherein said optical body comprises aplurality of repeating units having the layer sequence ABCB.
 27. Theoptical body of claim 25 , wherein A, B, and C have optical thicknessratios f_(x) ^(a), f_(x) ^(b), and f_(x) ^(c), respectively, along axisx, wherein f_(x) ^(a) is about ⅓, f_(x) ^(b) is about ⅙, and f_(x) ^(c)is about ⅓; and wherein n_(x) ^(b)=(n_(x) ^(a)n_(x) ^(c))^(½) and/orwherein said polymeric layer A has an optical thickness ratio f_(y)^(a)=⅓ along said in-plane axis y, said polymeric layer B has an opticalthickness ratio f_(y) ^(b)=⅙ along said in-plane axis y, said polymericlayer C has an optical thickness ratio f_(y) ^(c)=⅓ along said in-planeaxis y, and n_(y) ^(b)=(n_(y) ^(a)n_(y) ^(c))^(½).